In recent years, offshore exploration and production of petroleum products has been extended into arctic and other ice-infested waters in such locations as northern Alaska and Canada. These waters are generally covered with vast areas of sheet ice 9 months or more out of the year. Sheet ice may reach a thickness of 5 to 10 feet or more, and may have a compressive or crushing strength in the range of about 200 to 1000 pounds per square inch. A still more severe problem encountered in arctic waters is the presence of larger masses of ice such as pressure ridges, rafted ice or floebergs. Larger ice masses may have a thickness of up to 50 feet such that they move along the subsea bottom and produce scour marks in the bottom of up to several feet deep. Sheet ice and larger ice masses impose very high forces on any stationary structures in their paths; thus, it is very possible that an offshore structure may be forced off location by an impinging ice mass.
The possibility that a bottom-supported structure might be forced off a well site does present some unusual problems with respect to the structure's blowout prevention system. On bottom-supported structures, a surface blowout preventer (BOP) stack is used to provide the necessary well control for sealing the well when an abnormal well pressure develops, and generally, the surface BOP stack is located just below the drill floor of the structure. As discussed above, if very large ice forces are imposed on the structure, there exists a possibility that the structure will be forced off location. If this should occur, the wellhead would be damaged and the surface BOP stack would be disconnected from the wellhead which would prevent any sort of well control.
It has been proposed heretofore that a conventional surface BOP stack be utilized with mudline casing suspension equipment, which is used routinely on jack-up rig operations. The casing suspension equipment is installed at a wellhead in a chamber in the subsea bottom to permit removal of the riser from the subsea bottom should the structure be forced off location, and in such an event, the wellhead is protected from damage by being located in the chamber. The suspension equipment may include a casing bridge plug, a hanger-safety valve assembly, both of which must be run to the wellhead when the structure is being forced off location, or a hydraulically operated ball valve located between the wellhead and the riser connector.
The system discussed above, however, is inadequate for at least two reasons. First, it takes too long to remove the riser. Second, no BOP equipment for well control is provided should the structure be forced off location. Accordingly, the present invention is directed to a blowout prevention system which is capable of very quick removal of the riser from the underwater bottom and which provides adequate BOP protection.